Tonometer

ABSTRACT

A tonometer, for determining intraocular pressure, has a body, which can be a transparent, substantially tubular body. A plunger is located within the body and a coil spring acts between the body and the plunger. A marker member is frictionally retained within the body and is displaced relative to an external scale on the body. In use, the tonometer is brought up against the eyelid of a closed eye and the body displaced relative to the head of the plunger, until the pressure is sufficient to create a pressure phosphene. The device is then removed and the displacement of the marker member, indicative of the applied pressure is read. This reading corresponds to the intraocular pressure. The device may include a return member for returning the marker member to a zero or rest position

This application is a CIP of U.S. application Ser. No. 08/652,045 filedMay 23, 1996 U.S. Pat. No. 5,836,873 Nov. 17, 1998.

FIELD OF THE INVENTION

This invention relates to an apparatus for and a method of measuringintraocular pressure in the human eye. This invention more particularlyrelates to an applanation tonometer for measuring such intraocularpressure in the human eye.

BACKGROUND OF THE INVENTION

Now, it is well known that excessive internal pressure within the humaneyeball is a component of glaucoma, a disease of the eye. This diseaseaccounts for a significant percentage of all blindness. Surveys haveshown it to be present and unrecognized in a significant number ofpeople, particularly people over the age of 40 and even more so forpeople over the age 50.

Now, it is also known that where the presence of glaucoma can beidentified at an early stage, damage to the eye and subsequent blindnesscan be arrested. Appropriate medication and surgery can serve to arrestthe progress of the disease so that useful vision is retained.

In view of the fact that glaucoma is widespread, numerous proposals havebeen made for measuring the internal eyeball pressure. Many of these arecomplex precision instruments, which are expensive, and which requireelaborate clinical settings for their operation. Typically, suchinstruments apply an amount of force to the eyeball, sufficient to allowan objective measurement of specific flattening (applanation) orindenting (indentation) of the surface of the eye. The amount of forcerequired to achieve a certain applanation or indentation is correlatedwith the intraocular pressure measured internally, and usually expressedin mm of mercury.

Conventionally, the clinical instrument involved has some element whichis applied directly to the cornea of the open eye to measure theapplanation or indentation of the cornea. In view of natural humanreflexes, this requires a topical anesthetic. The equipment is complexand costly and requires a trained and sophisticated technician tooperate it. Other proposals have been made, and the following patentslist proposals known to the applicant: U.S. Pat. Nos. 1,637,421;1,661,718; 2,656,715; 5,176,139; and 5,197,473; French Patent 2,542,603;and Russian Federation Patents 2,004,187 and 457,466.

The Lipschutz U.S. Pat. No. 1,637,421 is a pressure indicator. It is notconcerned with measuring eyeball pressure, but rather it is concernedwith applying pressure to other parts of the human body. It is based onthe well known phenomenon that sensitivity to pressure of an area of thebody is an indication of disease. More particularly, it relies on thefact that the progress of the disease is related to the sensitivity ofan associated area of the body. As such, it provides a device enablingthe pressure applied to a particular area to be measured, so thispressure can be correlated with the progress of the disease. No cleardirections are given, with regard to applying this technique to thehuman eye. Measuring pressure in the human eye presents unique anddifficult problems, as compared to other parts of the anatomy. As thehuman eye is sensitive and delicate, everyone has a strong, naturalreflex to close their eyes, if any attempt is made to touch the eye.This Lipschutz patent does not address this issue.

A hardness testing device is disclosed in Patent No. 1,661,718 which isof marginal relevance.

An ocular tension indicator is disclosed in the Tolman U.S. Pat. No.2,656,715. However, this requires the eyeball to be contacted. It reliesupon relative axial displacement of different components of known, setweight, to determine the pressure within the eye. As such, it appears tobe a delicate, precision instrument. Since it must contact the nakedeye, it cannot be used outside of a clinical setting.

The two Fedorov U.S. Pat. Nos. 5,176,139 and 5,197,473 disclose anocular tonometer and a related method. This relies on a somewhat uniquetechnique where a ball is permitted to fall freely onto aneyelid-covered cornea. The kinetic energy of the ball deforms a cornea.The amount of the ball rebound varies depending upon the amount ofintraocular pressure and this is judged against the height of the ballrebound. This technique would appear difficult to carry out, since itdepends upon judging the height of the ball rebound.

Russian Patent 457,466 discloses an intraocular pressure transducer.This relies upon a Hall effect generator. Weights determine thepenetration force of a plunger, whose displacement is sensed by the Halleffect generator with an output proportional to the displacement.Russian Patent 2,004,187 discloses an eye tonometer having a hollowcylindrical body with tips and working end face surfaces. It is notclear how this device is intended to work. In any event, it is againintended to be applied to the naked eyeball, which again would requirethe application of a topical anesthetic in a clinical setting.

Now, one of the problems with measuring intraocular pressure is that itcan vary during the course of the day, and even from hour to hour.Accordingly, it is highly desirable to provide some simple, inexpensivetechnique for measuring this pressure. This technique should enable anordinary person to measure the intraocular pressure within their eyes,without requiring complex expensive equipment, without requiringattendance at a clinic or the like, and without requiring the time ofhighly trained clinical staff.

SUMMARY OF THE PRESENT INVENTION

In accordance with a first aspect of the present invention, there isprovided an applanation tonometer, for measuring pressure within a humaneye, the tonometer comprising: a main body, which is generally tubular,defines a bore and includes a portion bearing a scale; a plungerslidably mounted within the bore of the main body and including a headat one end for contacting an eyelid and the other end of the plungerbeing retained within the main body, the head being sufficiently largethat, in use, an eyeball is flattened and subjected to applanation;spring biasing means acting between the plunger and the main body,biasing the head away from the main body; and a marker memberfrictionally retained within the bore of the body for displacementrelative to the sliding scale by the plunger, to indicate a maximum loadapplied to the plunger, wherein the scale bearing includes viewing meanspermitting the location of the marker member relative to the scale to beviewed from the exterior and wherein the scale means is the only meansin the apparatus for measuring a characteristic of eye.

Preferably, a first end of the body, adjacent the plunger, includes afirst radially inwardly extending lip means and the plunger at the otherend thereof includes a first radially outwardly extending projectionmeans, the first lip means and the first projection means beingdimensioned such as to retain the other end of the plunger within thebody.

Conveniently, the body is generally elongate and is molded from atransparent material.

The tonometer preferably includes a return member slidably mounted inthe bore and extending from a second end of the body, the return memberpermitting a user to displace the marker member.

In accordance with another aspect of the present invention, there isprovided a method of diagnosing the presence of abnormal pressure withinan eyeball of a subject, the method comprising the steps of:

(1) providing a tonometer, for measuring pressure within a human eye,the tonometer including a head at one end for contacting an eyelid, thehead being sufficiently large to cause, in use, flattening andapplanation of an eyeball, and indication means for indicating a maximumload applied to the head;

(2) placing the tonometer on one eyelid and pressing the tonometeragainst the eyelid, so as to apply pressure through the head of thetonometer and the eyelid to the eyeball causing applanation of theeyeball;

(3) when the subject notices a pressure phosphene, terminatingdisplacement of the body, noting the maximum load indicated by theindication means and removing the tonometer;

(4) determining the pressure within the eyeball from the maximum loadindicated by the indication means when the pressure phosphene isdetected.

The method can include providing a tonometer including: a main body; aplunger slidably mounted relative to the main body and having the headmounted on one end thereof for contacting an eyelid; spring biasingmeans acting between the plunger and the main body, biasing the headaway from the main body; a marker member frictionally retained by one ofthe main body and the plunger for displacement relative thereto; and ascale provided on one of said main body and the marker member forindicating the magnitude of the maximum load, wherein the method furthercomprises:

(i) ensuring further that the marker member is initially locatedadjacent the zero position on the scale;

(ii) after the subject notices pressure phosphene, reading the locationof the marker member relative to the scale.

Additionally, the method can include determining the intraocularpressure within the eyeball from the load applied.

A pressure phosphene is an apparent spot or glow of light, or an arc oflight, detected by the subject.

Advantageously, the tonometer includes a return member slidably mountedwithin the bore of the body and extending through the other end of thebore, and then step (iii) comprises displacing the return member todisplace the marker member to the zero position.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

For a better understanding of the present invention and to show moreclearly how it may be carried into effect, reference will now be made,by way of example, to the accompanying drawings, which show preferredembodiments of a tonometer of the present invention, and in which:

FIG. 1 is a side view and in partial section of a first embodiment of atonometer in use;

FIG. 2 is a perspective view of a second embodiment of a tonometer inaccordance with the present invention;

FIG. 3 is a plan view of the tonometer of FIG. 2;

FIG. 4 is a view along the section line IV—IV of FIG. 3;

FIG. 5 is a view along the section line V—V of FIG. 4; and

FIG. 6 is a view in the direction of arrow VI of FIG. 3.

DESCRIPTION OF PREFERRED EMBODIMENTS

A first embodiment of an applanation tonometer in accordance with thepresent invention is generally designated by the reference 10 in thedrawings. The tonometer 10 has a main body or housing 12. The body 12 isessentially tubular and is adapted to be gripped and held by a user. Forthis purpose, it can include a specific section molded or shaped toensure good gripping.

The right hand end of the body 12, as shown in the drawing, includes aninwardly turned lip 14, for retaining a plunger, as detailed below.Additionally, an outwardly extending flange 16 is provided, tofacilitate gripping of the device, and pressing of the device against ausers eye, as detailed below.

At the left hand end, as viewed in the drawings, the body 12 includes asecond inwardly extending lip 18.

On the outside of the body, there is a graduated scale 20 provided withnumerical markings, to indicate a force supplied by the device. At leastthis portion of the body 12 should be of uniform section, but it is notimportant that the whole of the body 12 be of uniform section.

The body 12 defines an internal bore 22. As noted, at least where thescale 20 is provided, the body 12 should have constant cross section, sothat at this location the bore 22 would similarly have constant crosssection. Here, a marker 24 is slidably mounted within the bore 22. Themarker 24 is formed from resilient material, and is dimensioned to be aslight interference to fit within the bore 22, so as to be frictionallyretained within the bore 22. The dimensions of the marker 24 should besuch as to securely retain it at any position within the bore 22adjacent the scale 20, while at the same time enabling it to be freelymoved by displacement of a plunger, as detailed below. In particular,the force required to overcome the frictional retention of the marker 24should not be so great as significantly to affect a force readingobtained. Further, at least the scale portion 20 of the body 12 must betransparent, or at least partially transparent, to permit the positionof the marker 24 to be seen.

Extending out from the main body 12 is a plunger or contact member 26.This plunger 26 comprises a shaft 28, an annular retaining protection30, and a head 32. The head or contact member 32 can have any desiredshape. It is preferred for it to present a flat, circular disk surface34, and the profile to the left of that, as viewed in the drawing, isnot critical.

The projection 30 and lip 14 are both generally annular. The annularprojection 30 is dimensioned so as to have a slightly greater diameterthan the internal diameter of the lip 14. The difference in the twodimensions is sufficiently small to enable the tonometer 10 to beassembled by simply pressing the projection 30 through the lip 14,displacing the lip 14 radially outwards. For this purpose, the end ofthe body 12, adjacent the lip 14 can be provided with two or moreaxially-extending slots, so that right hand end portions of the body 12,as viewed, can be displaced radially outwards. Correspondingly, theprojection 30, as shown, can have a conical surface 38 for abutting thelip 14 and a planar surface 40 for abutting the lip 14 once it'sassembled.

A helical coil spring 36 is mounted, for compression loading, around theshaft 28. One end of the spring 36 abuts the outside of the lip 14,while the other end of the spring 28 abuts one side of the head 32. Thevarious dimensions are such that, once assembled, the spring 36 is notimmediately in compression. There is a certain amount of loose play.This is taken up to bring the marker 24 adjacent the zero point on thescale 20, as detailed below. The force required to displace the marker24 is much less than any significant load applied by the spring 36.Then, as the marker 24 is displaced, the load on the head 32 is almostsolely that applied by the spring.

Now, the inner end of the plunger 24, indicated at 42 is planar andadapted to abut the marker 24, to displace it, without becoming attachedto it. For displacing the marker 24 in the other direction, a returnmember 44 is provided. This return member 44 has an annular retainingprojection 46. The annular retaining projection 46 and lip 18 interact,in essentially the same manner as the lip 14 and projection 30. Again,the left hand end of the body 12, as viewed in the drawings, can beformed to facilitate engagement of the return member 44. The returnmember 44 has an abutment surface 48 for displacing the marker 24. It isotherwise mounted for free sliding movement within the body 12. It mayoptionally be provided with a spring 49, to keep it in an extendedposition.

In use, it is first ensured that the marker 24 is adjacent the zeropoint on the scale 20. If necessary, the return member 44 is displacedinto the body 12 to push the marker 24 into this position.

Then, the head 32 is brought up against the eyelid of a closed eye, thiseyelid being indicated at 50. The head 32 is applied to the upper medialaspect of the eye, away from the cornea. This is done by the subject oruser turning the eye outerwards and slightly downwards. The mostconvenient area may differ from person to person. The user just graspsthe body 12 and places the end surface 34 against the eyelid 50. Withthe surface 34 abutting the eyelid 50, the main body 12 is displacedtowards the eyelid 50, with the user's fingers pressing against theflange 16 if desired. This displacement drives the plunger 26 into thebody 12, displacing the marker member 24 along the scale 20. Thisincreases the load on the head 32 imparted by the spring 28.

This displacement continues, increasing the pressure on the head 32until the user detects, within their eye, a spot or glow of light, or anarc of light, known as a pressure phosphene. The user then stopsdisplacement of the body 12 and removes the tonometer 10 from the eye.The spring 28 will then displace the plunger 26 out of the body 12.However, the marker 24 will be frictionally retained at a location onthe scale 20. This location will indicate the load or force at which thepressure phosphene occurred. This is indicative of the intraocularpressure within the eye.

Either a direct pressure reading can be indicated on the scale 20, orthe user can be provided with a table correlating the scale reading 20with intraocular pressure. In either case, standard clinical tests couldbe carried out to correlate the intraocular pressure measured by thedevice of the present invention, i.e. the pressure at which a pressurephosphene is detected, with the actual pressure within the eye. Thiscalibration is done against Goldmann applanation tonometry, which is acorrelated and standard test. This correlation allows for variations intissue rigidity in the eyelid and sclera and for these to becompensated.

It is realised that the normal pressure for different groups of peoplemay vary. Thus, the normal for certain people may be a low reading. Forsuch people, problematic pressure could be indicated as simply anaverage pressure.

To accommodate this, the pressure scale needs, in effect, to be variedfor different types of users. This can be achieved in various ways.Where the scale on the device itself is to be correlated with a table,then it is simply a matter of providing different tables appropriate fordifferent users. If a scale is provided with indicated pressurereadings, then different springs with different spring rates could beused, effectively to vary the pressure applied for different readings onthe scale. Another option may be to vary the diameter of the head whereit contacts the eyelid, and this could possibly be used in combinationwith different springs. Normally, the head 32 would have the same sizeand profile as a Goldmann tonometer, to facilitate comparison of thetwo. Then, before a device is given to a patient or user, the patientwould be tested to determine to which category or group of people he orshe belongs. Once determined, an appropriate device can be provided, forhome testing.

If the measured pressure is outside desired limits, then the user willbe instructed either to apply medication previously provided to the userand/or to see an appropriate medical specialist for the condition to bechecked further. For example, if the user has a known condition ofexcess intraocular pressure, for intraocular pressure within a certainexcess pressure range, the user could simply be instructed to takepreviously prescribed medication. However, if pressure even higher thanthis set pressure is detected, then the user could be instructed to seehis or her doctor.

As noted above, to return the marker 24 to the zero position, forfurther use, for example on the other eye, the return member 44 issimply grasped and pressed against the marker 24 to displace it to thezero position. Which end of the marker 24 represents zero is purelyarbitrary, and can be indicated in instructional material provided withthe device.

The tonometer of the present invention has the great advantage that itis simple and robust. It can be produced at very little cost. Itsgreatest advantage is that it can be used by an ordinary person orpatient, and does not require highly trained clinical personal. Moreimportantly, it can be used in any setting, and does not requireattendance at a clinic, hospital or the like.

Reference will now be made to FIGS. 2-6 which show a second, preferredembodiment of a tonometer in accordance with the present invention. Thissecond embodiment corresponds, in principle, to the first embodiment,but the individual components have been adapted to be molded readilyfrom plastic. Here, the tonometer 60 has a main body or housing 62. Aplunger 64 is mounted in one end of the body 62, and a return member 66with an integral marker is mounted in the other end, as detailed below.Each of these elements is described in turn below.

The main body 62 has a tubular central portion 68 which defines acentral bore and includes a slot 70, on one side of which is a scale 72.

The first end of the body includes a circular end portion 74 of smallerdiameter than the main central portion, so as to define an annular ledgeor lip 75. As shown in FIGS. 4 and 5, extending axially inwardly fromthe end portion 74, there is a pair of ribs 76, and the innermostportion of these ribs 76 form a continuous crossbar 78 extendingdiametrically across the body 62.

At the second end of the body 62, there is a recess 80 surrounded by ashield 82.

The plunger 64 is generally tubular and has a bore 88 and a planar endsurface 90, which continues into the tubular side wall of the plunger 64either through a rounded portion or a conical surface. The other end ofthe plunger 62 has two legs 92 separated by a slot 94, and an annularprojection 96 extends around the free ends of the legs 92, for engagingthe annular ledge 75 of the main body.

A helical spring 98 is provided in the bore 88 for biassing the plunger64 to an extended position. To fit the plunger 64 in position, thespring 98 is inserted into the bore 88 of the plunger, and the plungeris then inserted through the end portion 74. The annular projection 96has a conical or tapered end surface that serves to squeeze the legs 92together, so that they pass through the bore of the circular end portion74. When fully inserted, the legs 92 spring outwards, so that theannular projection 96 engages the annular ledge 75, to secure theplunger in position.

The return member 66 comprises a head 100, a pair of side legs 102 and aslide member 104 adapted to slide freely in the bore of the main body62.

The slide member 104 comprises a circular disk and a pair of shortprojections 106, which serve to abut the end of the plunger 64 in a zeroposition, as shown in FIG. 5, with the crossbar 78 between theprojections 106.

Each leg 102 comprises a first leg portion 108, a friction portion 110and a second leg portion 112. The second leg portions 112 are smallerthan the first leg portions 108, and serve mainly to attach the head 100to the rest of the return member 66. The first leg portions 108 are ofrelatively large dimensions and are formed so as to bias the frictionportions 110 radially outwards. Then, as shown in FIG. 5, the frictionportions 110 are pressed against the inside of the tubular centralportion 68, so as to frictionally retain the return member 66 inposition. The degree of friction is such that the return member 66 willnot tend to move during normal handling of the tonometer 60, but at thesame time the level of friction is not so high as to significantlyaffect the force required to displace the return member 66.

The second leg portions 112 are of smaller dimensions, and serveessentially to connect the head 100 to the rest of the return member 66.

The head 100, as shown, has a semicircular edge 114 on one side and arectangular edge portion 116 on the other side, corresponding to theprofile of the recess 80.

Also extending from the head 100 is an arm 118, at the free end of whichis a marker projection 120 provided with an indication arrow 122. Aforward end of the projection 120 is provided with an inclined face 124and a rear face 126 perpendicular to the axis of the device, for thereasons given below.

To insert the return member in position, it is simply slid along theaxis of the main body 62. The inclined face 124 engages the edge of theshield 82 and deflects the projection 120 radially inwards. The arm 118causes the projection 120 to spring outwards into the slot 70. In thisposition, the face 126 will prevent the return member from beingaccidentally removed.

The second embodiment is used in essentially the same manner as thefirst embodiment. The spring keeps the plunger 64 in a normally extendedposition, as it is in compression between the crossbar 78 and the end ofthe plunger 64. Before using the device, a user simply grasps the mainbody in one hand and with a finger presses down on the head 100, todisplace the return member 66 to the rest position, or ensure that it isin the rest position, in which the marker projection 120 is adjacent azero position on the scale 72. This rest position is shown in FIG. 3,and as can be seen, the recess 80 provides adequate access for a finger.

The tonometer is then pressed against an eye, as in the first embodimentand the plunger 64 and return member 66 displaced by pressing with afinger on the head 100 until pressure phosphenes are detected. Thetonometer is then removed, and the position of the marker member 104, asindicated by the position of the arrow 122 on the scale 72 is noted,this being an indication of a load or force at which the pressurephosphene occurred, this being indicative of the intraocular pressurewithin the eye.

While a preferred embodiment of the invention has been described, itwill be appreciated that numerous variations are possible, within thespirit of the present invention. For example, while in the firstembodiment the return member 44 has been described as separate from themarker 24, it could be attached thereto where the spring 49 is omitted.This could be achieved by providing the marker 24 as a disk of resilientmaterial having a central bore, with the disk of material mounted on anend of the return member 44, provided with a collar to retain the markeron the return member. The return member could then simply be molded inplastic so as to be a free sliding fit, and be carried with the markeras it is displaced.

For both embodiments, it is preferred for the main body, the plunger andthe return member all to be molded from a plastic material. The scaleportion at least of the body should be transparent, or at least permitthe marker to be viewed, so that the marker can be viewed through thebody against the scale. Conveniently, the whole body is simply formedfrom transparent plastic material, of the type commonly used forsyringes and the like, or has a slot.

The marker member could be carried by either one of the main body 12 andthe plunger 26, with the scale provided by whichever element does notcarry the marker member.

What is claimed is:
 1. An applanation tonometer, for measuring pressurewithin a human eye, the tonometer comprising: a main body, which isgenerally tubular, defines a bore and includes a portion bearing ascale; a plunger slidably mounted within the bore of the main body,extending outwardly from a first end of the main body and including ahead at one end for contacting an eyelid and the other end of theplunger being retained within the main body, the head being sufficientlylarge that, in use, an eyeball is flattened and subject to applanation;spring biasing means acting between the plunger and the main body,biasing the head away from the main body; and a marker memberfrictionally retained within the bore of the body for displacementrelative to the scale by the plunger, to indicate a maximum load appliedto the plunger, wherein the scale bearing portion of the body includesviewing means permitting the location of the marker member relative tothe scale to be viewed from the exterior, and wherein the scale means isthe only means in the apparatus for measuring a characteristic of theeye.
 2. A tonometer as claimed in claim 1, wherein the body at the firstend, adjacent the plunger, includes a first radially inwardly extendinglip means and the plunger at the other end thereof includes a firstradially outwardly extending projection means, the first lip means andthe first projection means being dimensioned such as to retain the otherend of the plunger within the body.
 3. A tonometer as claimed in claim2, wherein the first projection means comprises an annular projectionhaving a generally conical surface, reducing in diameter in a directionaway from the one end of the plunger, and a planar surface, the conicalsurface serving to displace the first retaining lip means radiallyoutwards to permit insertion of the plunger during assembly of thetonometer and the planar surface being dimensioned to abut the firstretaining lip means to retain the one end of the plunger within thebody.
 4. A tonometer as claimed in claim 3, wherein the spring biasingmeans comprises a helical coil spring provided between the plunger andabutting the body.
 5. A tonometer as claimed in claim 4, wherein thebody is generally elongate and is molded from a transparent material. 6.A tonometer as claimed in claim 4, which includes a return memberslidably mounted in the bore and extending from a second end of thebody, the return member permitting a user to displace the marker member.7. A tonometer as claimed in claim 6, wherein the second end of the bodyincludes a second radially extending lip means and the return memberincludes one end, located within the body and including a secondradially outwardly extending annular projection means, the second lipmeans and the second annular projection means being adapted to retainthe return member within the body.
 8. A tonometer as claimed in claim 7,wherein the marker member is secured to the return member for movementtherewith.
 9. A tonometer as claimed in claim 6, wherein the markermember is integral with the return member, and the return memberincludes means for retaining the return member within the main body. 10.A tonometer as claimed in claim 9, wherein the main body includes anelongate slot and the scale is provided alongside the elongate slot andwherein the marker member comprises a marker projection secured to thereturn member and located in the slot, the marker projection includingan end face for abutting an end of the slot to prevent removal of themarker member.
 11. A tonometer as claimed in claim 10, wherein themarker projection is mounted on the end of a resilient arm and includesan inclined face, enabling radially inward displacement of the markerprojection, to facilitate insertion of the return member and the markermember into the main body.
 12. A tonometer as claimed in claim 9,wherein the return member includes a sliding member adapted for freesliding movement within the bore of the main body, a head at the otherend thereof, for manual engagement by a user to displace the returnmember, and leg means connecting the head to the sliding member, whichleg means include friction means resiliently biassed against the mainbody, so as to frictionally retain the return member in position.
 13. Atonometer as claimed in claim 12, wherein the leg means comprise a pairof oppositely located legs, each of which legs comprises a first legportion extending from the sliding member, a second leg portionextending from the head, and a friction portion between the first andsecond leg portions.
 14. A tonometer as claimed in claim 13, wherein thefirst leg portions are larger than the second leg portions.
 15. Atonometer as claimed in claim 12, wherein the main body includes arecess and a shield means at the second end of the main body,dimensioned such that for all positions of the marker member along thescale, the head of the return member is located within the recess andthe recess is of sufficient length to permit the head of the returnmember to be displaced along the recess to return the marker member to azero position on the scale.
 16. A tonometer is claimed in claim 15,wherein the shield projects outwardly from the main body and wherein thehead comprises a semicircular portion and a rectangular portion, whichis located within the recess.
 17. A method of diagnosing the presence ofabnormal pressure within an eyeball of a subject, the method comprisingthe steps of: (1) providing a tonometer, for measuring pressure within ahuman eye, the tonometer including a head at one end for contacting aneyelid, the head being sufficiently large to cause, in use, flatteningand applanation of an eyeball, and indication means for indicating amaximum load applied to the head; (2) placing the tonometer on oneeyelid and pressing the tonometer against the eyelid, so as to applypressure through the head of the tonometer and the eyelid to the eyeballcausing applanation of the eyeball; (3) when the subject notices apressure phosphene, terminating displacement of the body, noting themaximum load indicated by the indication means and removing thetonometer; and (4) determining the pressure within the eyeball from themaximum load indicated by the indication means when the pressurephosphene is detected.
 18. A method as claimed in claim 17, whichincludes providing a tonometer including: a main body; a plungerslidably mounted relative to the main body and having the head mountedon one end thereof for contacting an eyelid; spring biasing means actingbetween the plunger and the main body, biasing the head away from themain body; a marker member frictionally retained by one of the main bodyand the plunger for displacement relative thereto; and a scale providedon one of said main body and the marker member for indicating themagnitude of the maximum load, wherein the method Further comprises: (i)ensuring that the marker member is initially located adjacent the zeroposition on the scale; (ii) after the subject notices a pressurephosphene, reading the location of the marker member relative to thescale.
 19. A method as claimed in any one of claims to 18, whichincludes determining the intraocular pressure from the load applied. 20.A method as claimed in claim 18, which includes the following additionalstep: (iii) subsequently returning the marker member to the zeroposition.
 21. A method as claimed in claim 20, wherein the tonometerincludes a return member slidably mounted within the bore of the bodyand extending through the other end of the bore, wherein step (iii)comprises displacing the return member to displace the marker member tothe zero position.
 22. A method as claimed in claim 20 or 21 whichcomprises applying the head of the tonometer to an upper medial aspectof the eye, away from the cornea.
 23. A method of obtaining informationconcerning the pressure within the eyeball of a subject, the methodcomprising the step of determining the load on an eyeball when apressure phosphene is detected by the subject.
 24. The method of claim23 further including the step of applying the load to an eyelid, so asto apply pressure through the eyelid of the subject to the eyeball. 25.The method of claim 23 further including the step of determining thepressure within the eyeball from the load applied to the eyelid when thepressure phosphene is detected.
 26. A method as claimed in claim 25,which includes progressively increasing the load applied to the eyeliduntil the pressure phosphene is detected, terminating increase in theload when the pressure phosphene is detected, and noting the largestload applied to the eyelid as the load causing onset of the pressurephosphene.
 27. A method as claimed in any one of claims 23, 24, 25, and26, which includes applying the load with an applanation tonometer. 28.A method as claimed in claim 27, which includes determining theintraocular pressure from the applied load.
 29. A method as claimed inany one of claims 23, 24, 25, and 26, which includes determining theintraocular pressure from the applied load.